In a conventional cellular telephone, a modem provides a stream of complex, digital baseband samples to a transmitter, where the baseband samples are represented by real components and imaginary components (e.g., I and Q components). Along a digital portion of the transmitter lineup, the real components are processed along a first channel (a “real” channel), and the imaginary components are processed along a second channel (an “imaginary” channel), which is parallel to the first channel. The digital processing along either channel may include multiplexing, filtering, power control, and up-sampling processes, among other things. After converting the digitally-processed real and imaginary components to the analog domain (e.g., using digital-to-analog converters), the resulting analog signals corresponding to the real and imaginary channels are filtered, in parallel, along a subsequent, analog portion of the transmitter lineup. The parallel, filtered analog signals are then modulated to produce a single-channel, analog radio frequency (RF) signal. The RF signal is then amplified and radiated onto the air interface.
Ideally, along the analog portion of the transmitter lineup in which the real and imaginary signal components are processed along parallel channels (e.g., portions of the digital-to-analog converter, analog filter, and modulator), the circuit elements along one channel would be perfectly matched with corresponding circuit elements along the other, parallel channel. In actuality, however, the corresponding circuit elements along the real and imaginary channels are likely to have slight or relatively significant operational differences from each other due to manufacturing process variations and geometry differences, among other things. These differences may produce non-negligible amplitude differences (“amplitude-IQ-imbalances”) and phase differences (“phase-IQ-imbalances”) between the real and imaginary signals that are processed along the parallel channels.
Non-negligible amplitude-IQ-imbalances and phase-IQ-imbalances may be imposed by transmitters that are adapted to implement 2G (second generation), 2.5G (2.5 generation), 3G (third generation), and/or other wireless communication technologies. The characteristics of the modulation techniques performed in these conventional transmitters are such that factory calibration procedures (e.g., performed during manufacture) may be sufficient to provide adequate transmitter IQ imbalance correction across typical ranges of the device's operational parameters (e.g., transmit frequencies, battery charge, temperature, and so on). However, these factory calibration procedures tend to be time consuming, and therefore they do increase manufacturing time and cost.
More recent 4G (fourth generation) wireless communication technologies, however, propose to implement modulation techniques in which factory calibration may be impractical (e.g., too time-consuming to perform, and thus too costly). More particularly, at least some devices implementing 4G technologies may use OFDM (Orthogonal Frequency Division Multiplexing) for digital, multi-carrier modulation. With OFDM, a large number of closely-spaced, orthogonal sub-carriers are used to carry data within a frequency band. Within the band, each sub-carrier is positioned at a distinct offset frequency from a substantially band-centered carrier frequency. Data to be transmitted are divided into multiple parallel data streams (i.e., one for each sub-carrier), and the data stream corresponding to each sub-carrier is modulated with a conventional modulation scheme (e.g., QAM (Quadrature Amplitude Modulation) or PSK (Phase Shift Keying)). Effective factory calibration may necessitate calibration procedures for some or all sub-carriers associated with some or all carrier frequencies within the operational bandwidth. Accordingly, factory calibration for 4G devices would be even more time consuming than factory calibration procedures for 2G, 2.5G, and 3G devices. Because this would significantly increase manufacturing time and cost for 4G devices, factory calibration for 4G devices is an undesirable approach to providing transmitter IQ imbalance correction.
As an alternative, transmitter IQ imbalances may be left uncorrected. However, in devices in which modulation techniques such as those associated with 4G technologies are implemented, non-negligible transmitter IQ imbalances, left uncorrected, may result in unacceptably poor image rejection at some offset frequencies within the operational bandwidth. Accordingly, methods and apparatus are desired for measuring and correcting for transmitter IQ imbalances in wireless devices in which such imbalances may result in unacceptably degraded signal quality (e.g., wireless devices in which OFDM is performed). Additionally, it is desired that such methods and apparatus do not include time consuming factory calibration procedures.